Welcome

, I bought a 16 with 60k mi on the clock, granted i know the history on this truck (family) all updates had or have been done and got the letter/card recently that i may me eligible for the settlement deal, if i get it that'd be fine if not no big deal, but i have acquired all the parts to tune mine up, and will be doing it sometime soon regardless....as far as what to check, check trucks service record if available car fax if available, and the stuff i would look at myself would be the coolant tank mounted out towards the front of the truck, look at two things, the color of the coolant, should look pink/purple and the level, then i would look at the oil, just to know there was no water/coolant going places it should not be, on the level, make sure the coolant level is at the appropriate level (it is marked pretty clearly) secondly if the coolant level is below the seem (middle of the tank) i have heard and read it could be a sign of trouble, when running truck i'd monitor the coolant temp and oil temp, and if it is on a lot (non-dealer) drive it somewhere and try to engage the cruise below 25mph if it works under 25, it typically (if i understand that part right) indicates the truck has been tuned, personally i would rather have the tuned variety over an untuned simply for the reduced soot level produced by the factory ECM settings and the boondoggle EGR junk

some states have limited enforcement and some are hard to get around, however if you are in one of those states that has a visual and functionality check GDE has an EPA compliant tune that is getting really good review with greatly reduced soot levels
good luck, i like mine, but as mentioned, i bought from family for what dealer was offering for trade in value, so i feel i got a really good deal,
if any of this info is deemed "inaccurate" by other member of this forum feel free to correct me, cause i'm still learning too

, these were just things that had been mentioned to me, during my initial purchase